This is due to a problem impacting Windows XP. Our FireInspector confirmed that the 1394b ports are operating in S100 (100 mb/s) mode. They are never transitioning to S400, like they should. Microsoft has acknowledged that this is a problem, and they have fixed it with KB885222 for XP SP 2, and KB955408 for SP 3.
This can occur on system running Windows XP SP2 or XP SP3 because of a reduction in throughput on the 1394 bus. To correct this issue, download and install the Windows hotfix below.